Previous results have suggested that the rate of cell division in cultured cells is determined by the rate of accumulation of proteins required in the G1 phase of the cell cycle, with the rate of protein synthesis having the active control function when the culture growth rate is changed. This hypothesis will be tested in human diploid fibroblasts, normal malignant human epithelial cells, and two rapidly growing cell lines. The hypothesis implies a reciprocal-normal distribution of generation times in continuously proliferating populations; and it predicts that when the culture growth rate is changed by a change in extra-cellular conditions, the new rate of movement through the G1 phase and the new rate of protein synthesis will be correlated. The hypothesis will be tested by determining (1) if a reduction in the culture growth rate is accompanied by reversible withdrawal of cells from the cell cycle, as would be expected from reciprocal-normal kinetics, (2) whether the G1 times of siblings are correlated, (3) what quantitative relationship is maintained between the rate of protein synthesis and the rate of passage through the G1 phase, and (4) if the kinetics of growth stimulation are consistent with the hypothesis.